This invention is concerned generally with the production of a flexible, direct-imaging lithographic printing plate which is highly effective and yet more economical than most known lithographic plates. The particular thrust of the invention is to the achievement of a direct imaging plate which is to be used in offset printing and which can be discarded after one use, if desired because it is so economical to make.
Lithographic printing is a very old and well-known process in which the image is applied to a surface where the non-imaged portion of the surface is rendered hydrophilic while the image is hydrophobic. When inked with greasy inks, the image attracts the ink while the remainder of the surface repels it. The paper receptor is pressed against the surface and picks up only the inked image.
Thus, lithography is a printing process in which a planographic method is used, that is, the printing image and the non-printing areas all lie in substantially the same plane.
Offset lithography is probably the most important method of printing today. The principle is that ink is offset first from the plate to a rubber blanket and then from the blanket to the paper receptor. There may be an intervening metal drum instead of a rubber blanket. When the printing plate is made, the printing image is rendered hydrophobic, i.e., repellant to water but also attractive to grease. The non-printing areas are rendered just the opposite, that is, hydrophilic. On the press the plate is mounted on a plate cylinder which, as it rotates, comes into contact successively with rollers wet by a water or dampening solution and rollers wet by grease-based ink. The dampening solution wets the nonprinting areas of the plate and prevents the ink from wetting these areas. The ink wets the image areas which are transferred to the intermediate blanket cylinder. The paper picks up the image as it passes between the blanket cylinder and the impression cylinder.
Offset plates of conventional construction of the type expected to make many thousands of impressions are expensive to manufacture. Ink receptivity is accomplished by using inherently oleophilic (having an affinity for oil) resins or metals like copper or brass on the image areas. Water receptivity of the non-image areas is usually achieved by using hydrophilic metals like chromium, aluminum or stainless steel and this receptivity is maintained in platemaking and storage by using natural and synthetic gums such as for example, gum arabic.
All offset printing plates which are used for long runs exceeding several thousands of impressions are made by indirect imaging methods. The copy or intelligence is first required to be photographed onto silver halide film and the film negative then used to transfer the image to the printing plate. The transfer is accomplished in all such cases by means of photographic projection onto a coating which is light sensitive and carried by the plate. The negative is used to project the image onto the plate and the processes which follow for the development of the image on the plate vary. Thus, the plates are required to be stored in darkness until used or the light-sensitive coating applied just before use. This is true of the three types of long-run offset plates which are most popularly used today.
The invention herein provides an electrophotographic member which is imaged directly, toned and treated to become a printing plate. Treatment occurs in one or two simple steps that can be considered a single continuing step of several minor parts. The directness and simplicity of the process and the effectiveness of the product are not found in the graphic art, and yet, the durability of the plate of the invention rivals that of the metal plates in use today.
The three types of long run plates which are known at this time are surface, deep etch and bimetal. The surface plates are those in which a light-sensitive coating is exposed to a negative, developed etc. The process of achieving the plate requires many steps and treatments. On deep etch plates, after exposure to the negative, the coating in the image areas is removed and coppered chemically and/or lacquered and inked so they are ink receptive. The plate is usually aluminum and the process is quite involved and requires considerable skill. Bimetal plates are similar to deep etch in that the light sensitive coating is removed from the image areas but these areas consist of copper or brass.
The plates which are known are not transparent and hence cannot be viewed through optical projection. They cannot be readily corrected or added to.
In the case of the plate of the invention, the imaged plate may be washed off before treatment, corrected at that time, reimaged and additional images added with no problem.
One of the most important advantages of the invention is that the cost of a plate for printing is a small fraction of the cost of known lithographic offset plates.
It is known to make a printing plate utilizing zinc oxide as a photoconductive material. This is a somewhat complex structure in which the top layer is zinc oxide in a resin matrix with dyes for sensitizing, the second layer is paper substrate which has been treated with salt and glycerin along with a wet strength additive and the bottom layer is a conductive coating such as a water-soluble resin of the type used to render the paper substrate conductive. This plate has low sensitivity, low resolution, mediocre quality and can be used to make at most 3000 impressions. The material used to make the nonimaged areas hydrophilic is a potassium ferrocyanide.